Myosins are molecular motors found in all eukaryotic cells. The conventional myosins (myosins II) are two-headed, filament-forming, dimeric proteins that power diverse types of cell and subcellular movement. Native myosin is a hexamer, consisting of two molecules of myosin heavy chain and four molecules of myosin light chains. We will investigate the in vivo roles of the regulatory myosin light chains in the nematode Caenorhabditis elegans. Our approach is strongly genetic. We will isolate mutations that alter or eliminate the regulatory light chains in defined ways and study their in vivo effects on myosin-mediated events. We will investigate the functions of myosin and of the regulatory light chains in both muscle and nonmuscle cells. We will critically test the roles of myosin light chain phosphorylation by constructing mutant light chains that cannot be phosphorylated. Our approach combines genetic manipulation of the regulatory light chains with cell biological descriptions of the affected processes. Our long range goals are to understand how the regulatory light chains influence the activities of myosin, especially those pertaining to filament assembly or disassembly in both muscle and nonmuscle cells. The work is important for understanding the genetic origins of human muscular disease.